Sift the sugars and ground almonds together, then stir through the orange zest. Beat together the egg and egg yolk, then add to the sugar mixture, stirring well.
Knead briefly to ensure the marzipan is completely mixed and pliable, adding a little orange juice if the mix is too dry, or icing sugar if too wet.
Place the cake on a cake board and use small balls of marzipan to plug any holes in the cake. Now glaze the cake with a thin layer of marmalade. Dust your work surface with a little icing sugar, then roll out the marzipan in one direction, keeping the pressure even and turning a ¼ turn every few rolls.
When large enough (about 40cm in diameter for a 20cm cake), roll the marzipan onto the rolling pin and lift onto the cake. Smooth the top with your palms and continue down over the sides of the cake, smoothing out any folds as you go. Trim with a knife where the marzipan meets the board, then leave to dry overnight.
How long does homemade marzipan keep? Although homemade marzipan contains raw eggs, the amount of sugar, and lack of moisture, prevents bacteria growing when left at room temperature, so your cake should last for 1-2 months iced. The most important thing is that you don't scrimp on drying time - once you’ve covered your cake in marzipan it should be left to completely dry out for up to 3 days, before you then cover it with icing. If you want to make marzipan before you are ready to cover your cake, then wrap it well in cling film and keep in a fridge for up to a week - as wrapped in a ball it will still contain a little moisture. As it contains raw eggs it shouldn’t be given to anyone in an at-risk group, including pregnant women, the elderly, the unwell and the very young. You can make a cooked marzipan, which carries no risks. It is a little softer, and harder to work with, but a good substitute if you are at all worried.